Another oddity in handling of WCO constraints in postgres_fdw

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Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
Hi,

Commit 7086be6e3627c1ad797e32ebbdd232905b5f577f addressed mishandling of
WCO in direct foreign table modification by disabling it when we have
WCO, but I noticed another oddity in postgres_fdw:

postgres=# create table base_tbl (a int, b int);
postgres=# create function row_before_insupd_trigfunc() returns trigger
as $$begin new.a := new.a + 10; return new; end$$ language plpgsql;
postgres=# create trigger row_before_insupd_trigger before insert or
update on base_tbl for each row execute procedure
row_before_insupd_trigfunc();
postgres=# create server loopback foreign data wrapper postgres_fdw
options (dbname 'postgres');
postgres=# create user mapping for CURRENT_USER server loopback;
postgres=# create foreign table foreign_tbl (a int, b int) server
loopback options (table_name 'base_tbl');
postgres=# create view rw_view as select * from foreign_tbl where a < b
with check option;

So, this should fail, but

postgres=# insert into rw_view values (0, 5);
INSERT 0 1

The reason for that is: this is processed using postgres_fdw's
non-direct foreign table modification (ie. ForeignModify), but unlike
the RETURNING or local after trigger case, the ForeignModify doesn't
take care that remote triggers might change the data in that case, so
the WCO is evaluated using the data supplied, not the data actually
inserted, which I think is wrong.  (I should have noticed that as well
while working on the fix, though.)  So, I'd propose to fix that by
modifying postgresPlanForeignModify so that it handles WCO the same way
as for the RETURNING case.  Attached is a patch for that.  I'll add the
patch to the next commitfest.

Best regards,
Etsuro Fujita


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Re: Another oddity in handling of WCO constraints in postgres_fdw

Ashutosh Bapat
On Thu, Sep 28, 2017 at 5:45 PM, Etsuro Fujita
<[hidden email]> wrote:

> Hi,
>
> Commit 7086be6e3627c1ad797e32ebbdd232905b5f577f addressed mishandling of WCO
> in direct foreign table modification by disabling it when we have WCO, but I
> noticed another oddity in postgres_fdw:
>
> postgres=# create table base_tbl (a int, b int);
> postgres=# create function row_before_insupd_trigfunc() returns trigger as
> $$begin new.a := new.a + 10; return new; end$$ language plpgsql;
> postgres=# create trigger row_before_insupd_trigger before insert or update
> on base_tbl for each row execute procedure row_before_insupd_trigfunc();
> postgres=# create server loopback foreign data wrapper postgres_fdw options
> (dbname 'postgres');
> postgres=# create user mapping for CURRENT_USER server loopback;
> postgres=# create foreign table foreign_tbl (a int, b int) server loopback
> options (table_name 'base_tbl');
> postgres=# create view rw_view as select * from foreign_tbl where a < b with
> check option;
>
> So, this should fail, but
>
> postgres=# insert into rw_view values (0, 5);
> INSERT 0 1
>
> The reason for that is: this is processed using postgres_fdw's non-direct
> foreign table modification (ie. ForeignModify), but unlike the RETURNING or
> local after trigger case, the ForeignModify doesn't take care that remote
> triggers might change the data in that case, so the WCO is evaluated using
> the data supplied, not the data actually inserted, which I think is wrong.
> (I should have noticed that as well while working on the fix, though.)  So,
> I'd propose to fix that by modifying postgresPlanForeignModify so that it
> handles WCO the same way as for the RETURNING case.  Attached is a patch for
> that.  I'll add the patch to the next commitfest.
>

Enforcing WCO constraints imposed by the local server on the row/DML
being passed to the foreign server is fine, but trying to impose them
on the row being inserted/updated at the foreign server looks odd. May
be we should just leave this case as it is. I am comparing this case
with the way we handle constraints on a foreign table.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company


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Re: Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
On 2017/10/03 18:16, Ashutosh Bapat wrote:
> Enforcing WCO constraints imposed by the local server on the row/DML
> being passed to the foreign server is fine, but trying to impose them
> on the row being inserted/updated at the foreign server looks odd. May
> be we should just leave this case as it is. I am comparing this case
> with the way we handle constraints on a foreign table.

Hmm, I think that would be okay in the case where WCO constraints match
constraints on the foreign table, but I'm not sure that would be okay
even in the case where WCO constraints don't match?  Consider:

create table bt (a int check (a % 2 = 0));
create foreign table ft (a int check (a % 2 = 0)) server loopback
options (table_name 'bt');
create view rw_view_2 as select * from ft where a % 2 = 0 with check option;

In that case the WCO constraint matches the constraint on the foreign
table, so there would be no need to ensure the WCO constraint locally
(to make the explanation simple, we assume here that we don't have
triggers on the remote end).  BUT: for another auto-updatable view
defined using the same foreign table like this:

create view rw_view_4 as select * from ft where a % 4 = 0 with check option;

how is the WCO constraint (ie, a % 4 = 0) ensured remotely, which is
different from the constraint on the foreign table (ie, a % 2 = 0)?
Maybe I'm missing something, though.

Best regards,
Etsuro Fujita



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Re: Another oddity in handling of WCO constraints in postgres_fdw

Ashutosh Bapat
On Wed, Oct 4, 2017 at 3:45 PM, Etsuro Fujita
<[hidden email]> wrote:

> On 2017/10/03 18:16, Ashutosh Bapat wrote:
>>
>> Enforcing WCO constraints imposed by the local server on the row/DML
>> being passed to the foreign server is fine, but trying to impose them
>> on the row being inserted/updated at the foreign server looks odd. May
>> be we should just leave this case as it is. I am comparing this case
>> with the way we handle constraints on a foreign table.
>
>
> Hmm, I think that would be okay in the case where WCO constraints match
> constraints on the foreign table, but I'm not sure that would be okay even
> in the case where WCO constraints don't match?  Consider:
>
> create table bt (a int check (a % 2 = 0));
> create foreign table ft (a int check (a % 2 = 0)) server loopback options
> (table_name 'bt');
> create view rw_view_2 as select * from ft where a % 2 = 0 with check option;
>
> In that case the WCO constraint matches the constraint on the foreign table,
> so there would be no need to ensure the WCO constraint locally (to make the
> explanation simple, we assume here that we don't have triggers on the remote
> end).  BUT: for another auto-updatable view defined using the same foreign
> table like this:
>
> create view rw_view_4 as select * from ft where a % 4 = 0 with check option;
>
> how is the WCO constraint (ie, a % 4 = 0) ensured remotely, which is
> different from the constraint on the foreign table (ie, a % 2 = 0)? Maybe
> I'm missing something, though.

Just like the local constraints on a foreign table are not ensured on
remote table (unless user takes steps to make that sure), WCO defined
locally need not be (and probably can not be) ensured remotely. We can
check whether a row being sent from the local server to the foreign
server obeys WCO, but what foreign server does to that row is beyond
local server's scope.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company


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Re: Another oddity in handling of WCO constraints in postgres_fdw

Robert Haas
On Wed, Oct 4, 2017 at 6:40 AM, Ashutosh Bapat
<[hidden email]> wrote:
> Just like the local constraints on a foreign table are not ensured on
> remote table (unless user takes steps to make that sure), WCO defined
> locally need not be (and probably can not be) ensured remotely. We can
> check whether a row being sent from the local server to the foreign
> server obeys WCO, but what foreign server does to that row is beyond
> local server's scope.

But I think right now we're not checking the row being sent from the
local server, either.  The WCO that is being ignored isn't a
constraint on the foreign table; it's a constraint on a view which
happens to reference the foreign table.  It seems quite odd for the
"assume constraints are valid" property of the foreign table to
propagate back up into the view that references it.

--
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EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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Re: Another oddity in handling of WCO constraints in postgres_fdw

Ashutosh Bapat
On Wed, Oct 4, 2017 at 5:32 PM, Robert Haas <[hidden email]> wrote:

> On Wed, Oct 4, 2017 at 6:40 AM, Ashutosh Bapat
> <[hidden email]> wrote:
>> Just like the local constraints on a foreign table are not ensured on
>> remote table (unless user takes steps to make that sure), WCO defined
>> locally need not be (and probably can not be) ensured remotely. We can
>> check whether a row being sent from the local server to the foreign
>> server obeys WCO, but what foreign server does to that row is beyond
>> local server's scope.
>
> But I think right now we're not checking the row being sent from the
> local server, either.

Didn't 7086be6e3627c1ad797e32ebbdd232905b5f577f fix that?

> The WCO that is being ignored isn't a
> constraint on the foreign table; it's a constraint on a view which
> happens to reference the foreign table.  It seems quite odd for the
> "assume constraints are valid" property of the foreign table to
> propagate back up into the view that references it.
>

The view with WCO is local but the modification which violates WCO is
being made on remote server by a trigger on remote table. Trying to
control that doesn't seem to be a good idea, just like we can't
control what rows get inserted on the foreign server when they violate
local constraints. I am using local constraints as an example of
precedence where we ignore what's happening on remote side and enforce
whatever we could enforce locally. Local server should make sure that
any rows sent from local server to the remote server do not violate
any local WCO. But once it's handed over to the foreign server, we
shouldn't worry about what happens there. That behaviour is ensured by
the above commit, isn't it?  I am not suggesting that we use local
constraints to enforce WCO or something like that.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company


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Re: Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
On 2017/10/04 21:28, Ashutosh Bapat wrote:

> On Wed, Oct 4, 2017 at 5:32 PM, Robert Haas <[hidden email]> wrote:
>> On Wed, Oct 4, 2017 at 6:40 AM, Ashutosh Bapat
>> <[hidden email]> wrote:
>>> We can
>>> check whether a row being sent from the local server to the foreign
>>> server obeys WCO, but what foreign server does to that row is beyond
>>> local server's scope.
>>
>> But I think right now we're not checking the row being sent from the
>> local server, either.

We don't check the row *before* sending it to the remote server, but
check the row returned by ExecForeignInsert/ExecForeignUpdate, which is
allowed to have been changed by the remote server.  In postgres_fdw, we
currently return the data actually inserted/updated if RETURNING/AFTER
TRIGGER present, but not if WCO only presents.  So, for the postgres_fdw
foreign table, WCO is enforced on the data that was actually
inserted/updated if RETURNING/AFTER TRIGGER present and on the original
data core supplied if WCO only presents, which is inconsistent behavior.

> Didn't 7086be6e3627c1ad797e32ebbdd232905b5f577f fix that?

No.  The commit addressed another issue.

>> The WCO that is being ignored isn't a
>> constraint on the foreign table; it's a constraint on a view which
>> happens to reference the foreign table.  It seems quite odd for the
>> "assume constraints are valid" property of the foreign table to
>> propagate back up into the view that references it.

Agreed.

> The view with WCO is local but the modification which violates WCO is
> being made on remote server by a trigger on remote table. Trying to
> control that doesn't seem to be a good idea, just like we can't
> control what rows get inserted on the foreign server when they violate
> local constraints. I am using local constraints as an example of
> precedence where we ignore what's happening on remote side and enforce
> whatever we could enforce locally. Local server should make sure that
> any rows sent from local server to the remote server do not violate
> any local WCO.

Seems odd (and too restrictive) to me too.

Best regards,
Etsuro Fujita



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Re: Another oddity in handling of WCO constraints in postgres_fdw

Kyotaro HORIGUCHI-2
At Thu, 5 Oct 2017 18:08:50 +0900, Etsuro Fujita <[hidden email]> wrote in <[hidden email]>

> On 2017/10/04 21:28, Ashutosh Bapat wrote:
> > On Wed, Oct 4, 2017 at 5:32 PM, Robert Haas <[hidden email]>
> > wrote:
> >> On Wed, Oct 4, 2017 at 6:40 AM, Ashutosh Bapat
> >> <[hidden email]> wrote:
> >>> We can
> >>> check whether a row being sent from the local server to the foreign
> >>> server obeys WCO, but what foreign server does to that row is beyond
> >>> local server's scope.
> >>
> >> But I think right now we're not checking the row being sent from the
> >> local server, either.
>
> We don't check the row *before* sending it to the remote server, but
> check the row returned by ExecForeignInsert/ExecForeignUpdate, which
> is allowed to have been changed by the remote server.  In
> postgres_fdw, we currently return the data actually inserted/updated
> if RETURNING/AFTER TRIGGER present, but not if WCO only presents.  So,
> for the postgres_fdw foreign table, WCO is enforced on the data that
> was actually inserted/updated if RETURNING/AFTER TRIGGER present and
> on the original data core supplied if WCO only presents, which is
> inconsistent behavior.
>
> > Didn't 7086be6e3627c1ad797e32ebbdd232905b5f577f fix that?
>
> No.  The commit addressed another issue.
>
> >> The WCO that is being ignored isn't a
> >> constraint on the foreign table; it's a constraint on a view which
> >> happens to reference the foreign table.  It seems quite odd for the
> >> "assume constraints are valid" property of the foreign table to
> >> propagate back up into the view that references it.
>
> Agreed.
>
> > The view with WCO is local but the modification which violates WCO is
> > being made on remote server by a trigger on remote table. Trying to
> > control that doesn't seem to be a good idea, just like we can't
> > control what rows get inserted on the foreign server when they violate
> > local constraints. I am using local constraints as an example of
> > precedence where we ignore what's happening on remote side and enforce
> > whatever we could enforce locally. Local server should make sure that
> > any rows sent from local server to the remote server do not violate
> > any local WCO.
>
> Seems odd (and too restrictive) to me too.

Since WCO ensures finally inserted values, we can't do other than
acturally requesting for the values. So just merging WCO columns
to RETURNING in deparsed query is ok. But can't we concatenate
returningList and withCheckOptionList at more higher level?
Specifically, just passing calculated used_attr to
deparse(Insert|Update)Sql instead of returningList and
withCheckOptionList separately.  Deparsed queries anyway forget
the origin of requested columns.

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center



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Re: Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
On 2017/10/05 20:06, Kyotaro HORIGUCHI wrote:
> Since WCO ensures finally inserted values, we can't do other than
> acturally requesting for the values.

I think so too.

> So just merging WCO columns
> to RETURNING in deparsed query is ok. But can't we concatenate
> returningList and withCheckOptionList at more higher level?
> Specifically, just passing calculated used_attr to
> deparse(Insert|Update)Sql instead of returningList and
> withCheckOptionList separately.  Deparsed queries anyway forget
> the origin of requested columns.

We could do that, but I think that would need a bit more code to
postgresPlanForeignModify including changes to the deparseDeleteSql API
in addition to the deparse(Insert|Update)Sql APIs.  I prefer making high
level functions simple, so I'd vote for just passing withCheckOptionList
separately to deparse(Insert|Update)Sql, as proposed in the patch.

Best regards,
Etsuro Fujita



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Re: Another oddity in handling of WCO constraints in postgres_fdw

Robert Haas
In reply to this post by Ashutosh Bapat
On Wed, Oct 4, 2017 at 5:58 PM, Ashutosh Bapat
<[hidden email]> wrote:
> The view with WCO is local but the modification which violates WCO is
> being made on remote server by a trigger on remote table. Trying to
> control that doesn't seem to be a good idea, just like we can't
> control what rows get inserted on the foreign server when they violate
> local constraints.

I think that's a fair point.

--
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EnterpriseDB: http://www.enterprisedb.com
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Re: Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
(2017/11/01 11:16), Robert Haas wrote:
> On Wed, Oct 4, 2017 at 5:58 PM, Ashutosh Bapat
> <[hidden email]>  wrote:
>> The view with WCO is local but the modification which violates WCO is
>> being made on remote server by a trigger on remote table. Trying to
>> control that doesn't seem to be a good idea, just like we can't
>> control what rows get inserted on the foreign server when they violate
>> local constraints.
>
> I think that's a fair point.

For local constraints on foreign tables, it's the user's responsibility
to ensure that those constraints matches the remote side, so we don't
need to ensure those constraints locally.  But I'm not sure if the same
thing applies to WCOs on views defined on foreign tables, because in
some case it's not possible to impose constraints on the remote side
that match those WCOs, as I explained before.

Best regards,
Etsuro Fujita


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Re: Another oddity in handling of WCO constraints in postgres_fdw

Michael Paquier
On Fri, Nov 10, 2017 at 8:36 PM, Etsuro Fujita
<[hidden email]> wrote:
> For local constraints on foreign tables, it's the user's responsibility to
> ensure that those constraints matches the remote side, so we don't need to
> ensure those constraints locally.  But I'm not sure if the same thing
> applies to WCOs on views defined on foreign tables, because in some case
> it's not possible to impose constraints on the remote side that match those
> WCOs, as I explained before.

Moved to CF 2018-01.
--
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Re: Another oddity in handling of WCO constraints in postgres_fdw

Stephen Frost
In reply to this post by Etsuro Fujita
Greetings Etsuro, Robert, all,

* Etsuro Fujita ([hidden email]) wrote:

> (2017/11/01 11:16), Robert Haas wrote:
> >On Wed, Oct 4, 2017 at 5:58 PM, Ashutosh Bapat
> ><[hidden email]>  wrote:
> >>The view with WCO is local but the modification which violates WCO is
> >>being made on remote server by a trigger on remote table. Trying to
> >>control that doesn't seem to be a good idea, just like we can't
> >>control what rows get inserted on the foreign server when they violate
> >>local constraints.
> >
> >I think that's a fair point.
>
> For local constraints on foreign tables, it's the user's responsibility to
> ensure that those constraints matches the remote side, so we don't need to
> ensure those constraints locally.  But I'm not sure if the same thing
> applies to WCOs on views defined on foreign tables, because in some case
> it's not possible to impose constraints on the remote side that match those
> WCOs, as I explained before.
Reviewing this thread, I tend to agree with Etsuro and I'm not sure I
see where there's a good argument for having a foreign table under a
view behave differently than a local table under a view for WCO (which
is an option of the view- not about the table underneath it or if it's
local or remote).  I've not done a detailed review of the patch but it
seems pretty reasonable and pretty small.

Thanks!

Stephen

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Re: Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
(2018/01/17 22:00), Stephen Frost wrote:
> Reviewing this thread, I tend to agree with Etsuro and I'm not sure I
> see where there's a good argument for having a foreign table under a
> view behave differently than a local table under a view for WCO (which
> is an option of the view- not about the table underneath it or if it's
> local or remote).  I've not done a detailed review of the patch but it
> seems pretty reasonable and pretty small.

Thanks for the comments!

I noticed the patch doesn't apply.  Attached is a rebased patch.

Best regards,
Etsuro Fujita

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Re: Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
(2018/01/18 16:16), Etsuro Fujita wrote:
> Attached is a rebased patch.

I rebased the patch over HEAD and revised comments/docs a little bit.
Please find attached a new version of the patch.

Best regards,
Etsuro Fujita

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Re: Another oddity in handling of WCO constraints in postgres_fdw

a.zakirov
Hello,

On Wed, Feb 28, 2018 at 05:22:42PM +0900, Etsuro Fujita wrote:
> I rebased the patch over HEAD and revised comments/docs a little bit. Please
> find attached a new version of the patch.

I've reviewed the patch.

The code is good, clear and it is pretty small. There are documentation
fixes and additional regression tests.

Unfortunately the patch is outdated and it needs rebasing. Outdated
files are regression tests files.

After rebasing regression tests they pass.

--
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company

Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company
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Re: Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
Hi Arthur,

(2018/03/03 18:51), Arthur Zakirov wrote:

> On Wed, Feb 28, 2018 at 05:22:42PM +0900, Etsuro Fujita wrote:
>> I rebased the patch over HEAD and revised comments/docs a little bit. Please
>> find attached a new version of the patch.
>
> I've reviewed the patch.
>
> The code is good, clear and it is pretty small. There are documentation
> fixes and additional regression tests.
>
> Unfortunately the patch is outdated and it needs rebasing. Outdated
> files are regression tests files.
>
> After rebasing regression tests they pass.
I rebased the patch over HEAD.  Please find attached an updated patch.

Thank you for the review!

Best regards,
Etsuro Fujita

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Re: Another oddity in handling of WCO constraints in postgres_fdw

a.zakirov
On Mon, Mar 05, 2018 at 09:44:37PM +0900, Etsuro Fujita wrote:
> I rebased the patch over HEAD.  Please find attached an updated patch.

Thank you!

IMHO, it is worth to add more explaining comment into
deparseReturningList, why it is necessary to merge WCO attributes to
RETURNING clause. You already noted it in the thread. I think it could
confuse someone who not very familiar how RETURNING is related with WITH
CHECK OPTION.

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Arthur Zakirov
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Re: Another oddity in handling of WCO constraints in postgres_fdw

Etsuro Fujita
(2018/03/06 1:57), Arthur Zakirov wrote:
> IMHO, it is worth to add more explaining comment into
> deparseReturningList, why it is necessary to merge WCO attributes to
> RETURNING clause. You already noted it in the thread. I think it could
> confuse someone who not very familiar how RETURNING is related with WITH
> CHECK OPTION.

Agreed.  I added a comment to that function.  I think that that comment
in combination with changes to the FDW docs in the patch would help FDW
authors understand why that is needed.  Please find attached an updated
version of the patch.

Thanks for the comments!

Best regards,
Etsuro Fujita

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Re: Another oddity in handling of WCO constraints in postgres_fdw

a.zakirov
On Tue, Mar 06, 2018 at 08:09:50PM +0900, Etsuro Fujita wrote:
> Agreed.  I added a comment to that function.  I think that that comment in
> combination with changes to the FDW docs in the patch would help FDW authors
> understand why that is needed.  Please find attached an updated version of
> the patch.

Thank you.

All tests pass, the documentation builds. There was the suggestion [1]
of different approach. But the patch fix the issue in much more simple
way.

Marked as "Ready for Commiter".


1 - https://www.postgresql.org/message-id/20171005.200631.134118679.horiguchi.kyotaro%40lab.ntt.co.jp

--
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company

Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company
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